Like the Web, email, and other Internet communications tools, IP telephony can be secured. This fact is one of its biggest appeals over old-school telephone equipment. Security means enforcing system policy, recording instances of abuse for forensic and litigation purposes, encrypting or otherwise hiding sensitive information in transit, bolstering call-management systems' resilience to exploitive attacks and computer viruses, and securing the access perimeter of the VoIP network.
Security tools and enforcement practices for VoIP applications are the same, essentially , as those for other IP-based apps, because they run on the same network. The security objective of VoIP systems is largely the same as those of other IP-based systems: in a nutshell , preserve the operational status of the system.
There are many threats to this objective and many countermeasures to the threats. Policy enforcements points, like firewalls, protect lower layers of the network, while authentication systems like RADIUS and application proxies provide higher-layer security. This chapter describes how to secure and harden a VoIP server, the basics of DMZs, how to enable logging of VoIP traffic with iptables, how to tweak the logging configuration of Asterisk, and how to log and monitor VoIP network traffic.